Conventional induction motors maintain the full sine wave of voltage across the stator winding regardless of the load in the motor. In those cases where the load varies within wide limits e.g., when the motor is used for hoisting operations, most of the time the motor is not expanding its full rated load. In such cases, the iron losses in the stator are substantially the same when the motor is operating below full rated load as is the case when the motor is operating at full rated load; and, due to low power factor in such cases, the stator current is high and the copper losses are also substantial.
When a conventional polyphase induction motor is operating below its full rated load, a fraction of the sine wave of voltage or operation on a single phase would satisfy the actual load requirement imposed on the motor. Such cutting in part in the sine wave voltage or operating on a single phase would result in considerably less iron and copper losses and less heating of the stator. The resultant lower operating temperature further reduces the copper losses in the motor due to lowered ohmic resistance. These factors combine to effect a significant reduction in the energy which is consumed by the motor, with a consequent conservation in available energy sources and a reduction in operating costs.
The present invention is based upon a recognition of the foregoing factors, and provides a simple yet reliable mechanism operative to cause the electrical energy supplied to the stator and the stator flux density of a standard-unmodified-AC polyphase induction motor to become a function of its load demand at any given moment. The invention accomplishes this by permitting a greater or smaller portion of the sine wave of voltage from a power source to enter the stator as a function of the percentage of slip of the motor or by operating on a single phase. In other words, the sine wave of the voltage supplied to the motor's stator is modified to suit existing load conditions. This results in the reduction of iron and copper losses.